First of all, the intrusion formed on the machined face of the workpiece in the wire electric discharge machining will be described. FIG. 6 is an explanatory view showing a relative movement passage (hereinafter referred to as an electrode passage) between a wire electrode and the workpiece in making the wire electric discharge machining. In FIG. 6, reference numeral 1 denotes the workpiece, 2 denotes a machining configuration portion (hereinafter referred to as a configuration portion) on the electrode passage for machining the workpiece 1 in an intended contour shape, 3 denotes a point for starting the machining (hereinafter referred to as a machining start point) such as a machining start hole, and 4 denotes a runway passage portion (hereinafter referred to as a runway portion) of the electrode passage leading from the machining start point 3 to the configuration portion 2. Reference character O denotes a connection point (hereinafter referred to as an approach point) between the configuration portion 2 and the runway portion 4. FIG. 7 is an enlarged view near the approach point O of FIG. 6. The same numerals designate the same or like parts as in FIG. 6. In FIG. 7, reference numeral 5 denotes a wire electrode and 6 denotes an intrusion.
As well known, the wire electric discharge machining is a working method of cutting out the workpiece 1, using the wire electrode 5 like a jigsaw. As shown in FIG. 6, not only the configuration portion 2 but also the runway portion 4 are worked.
In the configuration portion 2 of the ordinary wire electric discharge machining, the path is spaced away from the machining configuration by a certain distance (hereinafter referred to as an offset amount) in consideration of the radius of wire electrode 5, discharge gap length, and finishing allowance, and the machining is made by moving the center of the wire electrode 5 along this offset path. That is, the wire electrode 5 is moved from the machining start point 3 through the runway portion 4, from the approach point O to the configuration portion 2, rounds the configuration portion 2, and again moved from the approach point O through the runway portion 4 back to the machining start point 3, as shown in FIG. 7. If the finish machining is needed, the wire electrode 5 is repetitively moved along the electrode passage where an offset amount is set up in succession for every machining electric discharge conditions for use.
By the way, when the wire electrode 5 is moved around the configuration portion 2 as previously described, at the time of rounding the configuration portion 2, the wire electrode only once passes the configuration portion 2 other than the approach point O, but passes the approach point O twice. When the wire electrode 5 passes the approach point O at the second time, the workpiece 1 to be machined has been already removed when the wire electrode 5 passes the approach point O at the first time, so that the discharge occurs on the already machined face, over-cutting the machined face. Accordingly, the intrusion 6 is formed on the machined face of the workpiece 1 near the approach point O, making the machining precision worse.
FIG. 8 is an explanatory view showing a constitution of the conventional wire electric discharge machining apparatus as disclosed in JP-A-4-189421, and explaining how to suppress the intrusion from being formed. In FIG. 8, reference numeral 1 denotes the workpiece, 5 denotes the wire electrode, 7 denotes wire electrode supplying means, 8 denotes working fluid supplying means, 9 denotes driving means, 10 denotes machining power supplying means, 11 denotes control means, and 12 denotes program automatic conversion means. FIG. 9 is an explanatory view showing the electrode passage in the wire electric discharge machining apparatus of FIG. 8. In FIG. 9, reference numeral 1 denotes the workpiece, 2 denotes the configuration portion, 3 denotes the machining start point, and 4A and 4B denote the runway portion. Reference sign OA denotes a first approach point, and OB denotes a second approach point.
The operation will be now described. The wire electrode supplying means 7 of FIG. 8 delivers the wire electrode 5 with an appropriate tension applied to the wire electrode 5 while being traveled at a predetermined rate. The working fluid supplying means 8 supplies a working fluid between the wire electrode 5 and the workpiece 1. The machining power supplying means 10 applies a pulsed voltage between the wire electrode 5 and the workpiece 1 to generate a discharge. The program automatic conversion means 12 extracts the approach point O from a program describing the electrode passage stored in the control means 11, based on the set information of the offset amount, thereby grasping the runway portion 4 and the configuration portion 2, when making the machining as shown in FIG. 6. Then, the first approach point OA and the second approach point OB are set up on the configuration portion 2, the paths of the runway portions are connected to two approach points (runway portions 4A and 4B), and the program is changed to delete the electrode passage in the configuration portion between two approach points (between the approach points OA and OB in FIG. 9), and stored in the control means 11 again.
That is, when the electrode passage near the approach point is as shown in FIGS. 6 and 7, the electrode passage is changed as shown in FIG. 9. The control means 11 drives the driving means 9 to relatively move the wire electrode 5 and the workpiece 1 along the changed electrode passage.
As above described, in the conventional wire electric discharge machining apparatus, the program automatic conversion means 12 provides the electrode passage program with the first approach point OA through which the wire electrode 5 passes in entering from the runway portion 4A to the configuration portion 2 and the second approach point OB through which it passes in returning from the configuration portion 2 to the runway portion 4B, and the program is changed not to pass the wire electrode 5 through the configuration portion between two approach points, suppressing the intrusion from being formed on the machined face of the workpiece 1.
With this conventional method, since the electrode passage program is changed to set up two approach points, there is a risk of causing a new interference between the wire electrode and the workpiece. Also, the workpiece may be over-cut in one part and under-cut in the other part in the configuration between two approach points, resulting in a problem that it is difficult to adjust the distance between the approach points.